Prep: Toast almonds in a dry pan over medium heat until fragrant and lightly browned; cool. Line a 5- by 9- by 3-inch loaf pan with plastic wrap, leaving some overhang. Spread cooled almonds across the bottom; set aside. Whip cream to sturdy peaks. Chill.

Whisk: Finely zest and juice the lemons. Measure 5 teaspoons zest and 1/2 cup juice into the bowl of an electric mixer. (Save remaining zest and juice for the berries.) Stir in 1 cup plus 2 tablespoons sugar, yolks and 1/4 teaspoon salt. Set bowl over a pan of simmering water, and whisk until 165 degrees, about 8 minutes. Pull pan off heat.

Mash: For berries, place blueberries into a large saucepan. Stir in 1/3 cup sugar, 1 tablespoon reserved lemon juice, 1 teaspoon reserved lemon zest and pinch of salt. Cook over medium-high heat, mashing many of the berries with the back of a slotted spoon, until saucy, 4 to 5 minutes. Stir in cornstarch slurry; let thicken, 1 minute. Pull pan off heat; stir in blackberries.

Serve: Peel plastic from top of semifreddo; invert onto a serving platter and remove all of plastic wrap. Using a heavy knife dipped in hot water and wiped dry, slice semifreddo into 8 (1-inch-thick slabs). Ladle about 1/3 cup warm berry sauce onto dessert plates. Lay 1 slab semifreddo on top of each and serve.